Evaluator cleanups

* Don't use isCurrent anymore; instead look up builds in the previous
  jobset evaluation.  (The isCurrent field is still maintained because
  it's still used in some other places.)

* To determine whether to perform an evaluation, compare the hash of
  the current inputs with the inputs of the previous jobset
  evaluation, rather than checking if there was ever an evaluation
  with those inputs.  This way, if the inputs of an evaluation change
  back to a previous state, we get a new jobset evaluation in the
  database (and thus the latest jobset evaluation correctly represents
  the latest state of the jobset).

* Improve performance by removing some unnecessary operations and
  adding an index.

src/lib/Hydra/Controller/Build.pm

@@ -538,7 +538,7 @@ sub clone_submit : Chained('build') PathPart('clone/submit') Args(0) {
     my %currentBuilds;
     my $newBuild = checkBuild(
         $c->model('DB'), $build->project, $build->jobset,
-        $inputInfo, $nixExprInput, $job, \%currentBuilds);
+        $inputInfo, $nixExprInput, $job, \%currentBuilds, undef);
 
     error($c, "This build has already been performed.") unless $newBuild;